Maintain div aspect ratio according to height

Question

I need to maintain the width of an element as a percentage of its height. So as the height changes, the width is updated.

The opposite is achievable by using a % value for padding-top, but padding-left as a percentage will be a percentage of the width of an object, not its height.

So with markup like this:

<div class="box">
  <div class="inner"></div>
</div>

I'd like to use something like this:

.box {
    position: absolute;
    margin-top: 50%;
    bottom: 0;
}
.inner {
    padding-left: 200%;
}

To ensure the box's aspect ratio is maintained according to it's height. The height is fluid because of it's % margin - as the window's height changes, the box's height will too.

I know how to achieve this with JavaScript, just wondering if there's a clean CSS-only solution?

Answer 1

You can use an image that has the desired proportions as to help with proportional sizing (images can be scaled proportionally by setting one dimension to some value and other to auto). The image does not have to be visible, but it must occupy space.

.box {
  position: absolute;
  bottom: 0;
  left: 0;
  height: 50%;
}
.size-helper {
  display: block;
  width: auto;
  height: 100%;
}
.inner {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background: rgba(255, 255, 153, .8);
}

<div class="box">
  <img class="size-helper" src="//dummyimage.com/200x100/999/000" width="200" height="100">
  <div class="inner">
    1. box has fluid height<br>
    2. img has 2:1 aspect ratio, 100% height, auto width, static position<br>
    2.1 it thus maintains width = 200% of height<br>
    2.2 it defines the dimensions of the box<br>
    3. inner expands as much as box
  </div>
</div>

In the above example, box, inner and helper are all same size.